This invention relates to a process for the preparation of cyanuric acid by heating a reaction solution of urea, biuret or mixtures thereof dissolved in a solvent while stripping the reaction mixture with a stripping gas. Netherlands Patent Application 69.10466 and corresponding U.S. Pat. No. 3,635,968 describe such a process wherein N-cyclohexyl pyrrolidone is employed as a solvent and an inert gas such as nitrogen or carbon dioxide is passed through the reaction mixture to remove ammonia which forms during the reaction.
A problem which exists in such known processes resides in the fact that the valuable by-product ammonia must be recovered from a gaseous mixture which of course is highly diluted with the inert gas. Such recovery methods entail undesirably high costs. Such is also true with regard to recovery of gaseous solvent which may be contained in the vented waste gas from the reaction mixture.
Ammonia can, for example, be washed from the gaseous mixture with water, but this mode results in obtaining a dilute ammonia solution which normally would necessitate further concentration for use. Ammonia can also be recovered by condensation, but such methods are most expensive owing to the fact that at normal pressures, ammonia exists as liquid only at below -30.degree. C. Of course, one may raise the condensation temperature of ammonia by increasing the pressure, but such steps require considerable compression energy.
Consequently, none of these methods for recovering ammonia from the vent-gas mixture, which contains large amounts of inert gas, have proven satisfactory.
When the stripping treatment is omitted from such processes, one obtains an unsuitable product which is highly contaminated with aminated by-products such as ammelide and ammeline. It is important to obtain cyanuric acid having a low content of aminated by-products since generally an aminated by-product content of over one percent is commercially unacceptable. Therefore, crude cyanuric acid having an aminated by-product content in excess of one percent by weight is generally purified by treatment with an aqueous solution of a strong acid to hydrolyze ammelide and ammeline to cyanuric acid. However, such a hydrolysis step is expensive and thus it would be desirable to avoid same if possible.
Accordingly, it is the primary object of the present invention to provide a process for producing cyanuric acid whereby by-product ammonia may be readily separated from the vent gases resulting from the reaction mixture.
It is a further object of the present invention to provide a process for preparing cyanuric acid having a low aminated by-product content.
These and other objects of the present invention will be readily apparent from the description which follows.